1. Field of the Invention
The present invention relates to a phase adjusting circuit for adjusting the phase of a signal to be transmitted, and an oscillator in which an oscillation loop circuit includes the phase adjusting circuit.
2. Description of the Related Art
A surface acoustic wave (SAW) oscillator using a SAW resonator as a resonant element ordinarily has a configuration in which a phase adjusting circuit including an inductor is connected in parallel to the SAW resonator in order to stabilize an oscillation frequency. A constant of this phase adjusting circuit is determined based on an oscillation frequency set beforehand when considering electrical characteristics of the SAW resonator.
All elements, such as SAW resonators, do not exhibit identical characteristics, but exhibit different characteristics in a set range of variations although the characteristics are substantially similar. Accordingly, when oscillators are constructed of the SAW resonators, a problem occurs in that each SAW resonator that is used changes the oscillation frequency of each oscillator.
Accordingly, as shown in FIG. 6, a SAW resonator for solving the above-described problem has been proposed. In the SAW resonator, the inductance of an inductor in a phase adjusting circuit connected in series to a resonant element can be changed (see, for example, Japanese Unexamined Patent Application Publication No. 2-306705).
FIG. 6 is a schematic perspective view of the configuration of the phase adjusting circuit of the related art (disclosed in Japanese Unexamined Patent Application Publication No. 2-306705).
As shown in FIG. 6, a phase adjusting circuit 100 has a shape defined by separating line electrodes 11 provided on one surface of a substrate at a predetermined point and bending the separated portions perpendicularly to the line electrodes 11 to form a pair of parallel end-open lines 12a and 12b. In the phase adjusting circuit 100, predetermined positions of the end-open lines 12a and 12b are set to be in conduction by a zero-ohm resistor 13. Accordingly, portions composed of the end-open line 12a, the zero-ohm resistor 13, and the end-open line 12b from the bending points of both lines define an inductor. By changing the mounting position of the zero-ohm resistor 13, the inductance is changed.
In the phase adjusting circuit 100 shown in FIG. 6, since intermediate points of the end-open lines 12a and 12b are set to be in conduction by the zero-ohm resistor 13, portions of the end-open lines 12a and 12b from the zero-ohm resistor 13 to their ends function as open stubs, thus causing an adverse effect on oscillation characteristics of the SAW oscillator. This makes it difficult to obtain a signal having an oscillation frequency having a value equal to that set beforehand. In addition, the distance between each point of connection to the zero-ohm resistor 13 and each end of the end-open lines 12a and 12b cannot be determined unless adjustment is performed after mounting the zero-ohm resistor 13. Thus, it is impossible to predict how the mounting position will cause an adverse effect on the circuit.
In addition, a variable range of the inductance is narrowed since the inductor is formed by only two parallel lines. Moreover, a large area is occupied by the inductor on the substrate which prevents a reduction in the size of the oscillator.